Moyes has a better transfer record, better record with youth players moving into the squad, has achieved far more in England, is younger, and would have a better chance at keeping Downing and others, in my opinion.
McLeish has some nice achievements that don't necessarily translate to the situation at hand, and he also has some shockers on the CV, such as leading Rangers to 3rd in Scotland and getting relegated with a couple different teams. It's probably unfair not to give him some credit for the Carling Cup (regardless of how easy the draw was and how lucky they were), for success in Europe at Rangers, and for repromoting Hibs and SHA after relegating each. It's also hard to tell just how much of the struggles at SHA were down to the board's interference and incompetence--however, it seems that last year's result was inexcusable regardless of any meddling based on the amount that Yeung invested in the net spend. And SHA allowed one more (or was it less?) goal than Villa last season--how much of McLeish's defensive reputation is staked onto lucking into loaning Hart and buying the central pairing from lower leagues (as well as negative football)? Hopefully he can tighten up our defence, but last season would suggest that the pieces may not be on the roster at present.
I will say that I originally saw McLeish's resignation by email as unprofessional, but it appears that it was turnabout as his chief scout had been fired in a similar manner. So at least that reservation has been removed.
But the biggest red flag of taking McLeish over similar (for argument's sake) candidates is the divisive nature of his appointment. McLeish has nowhere near the credentials to overcome the initial shock of his appointment for many supporters, especially coming freshly off a relegation. Funny when it happens, sad when you cast your fortunes on the architect. The board had to consider the effects of this hire, and I'm sure they think they have, but perhaps they were considering more of an internet furor rather than something with tangible effect. But every disappointing result, every young player that isn't getting enough time, every struggling starter that gets repeated chances, and every substitution made or unmade will carry that much more scrutiny unless and until McLeish puts up a lot of surprisingly good results. And although it would be nice if everyone could support him until he's proven himself one way or the other, it's unrealistic. I think most people have a strongly held opinion formed when they first heard of the likely appointment, and that opinion is going to be tough to completely change. Just my opinion.